The main case for this function is when s is not NULL and pwc is not
NULL. In this case, the mbtowc() function inspects at most n bytes
of the multibyte string starting at s, extracts the next complete
multibyte character, converts it to a wide character and stores it at
*pwc. It updates an internal shift state known only to the mbtowc()
function. If s does not point to a null byte ('\0'), it returns the
number of bytes that were consumed from s, otherwise it returns 0.
If the n bytes starting at s do not contain a complete multibyte
character, or if they contain an invalid multibyte sequence, mbtowc()
returns -1. This can happen even if n >= MB_CUR_MAX, if the
multibyte string contains redundant shift sequences.
A different case is when s is not NULL but pwc is NULL. In this
case, the mbtowc() function behaves as above, except that it does not
store the converted wide character in memory.
A third case is when s is NULL. In this case, pwc and n are ignored.
The mbtowc() function resets the shift state, only known to this
function, to the initial state, and returns nonzero if the encoding
has nontrivial shift state, or zero if the encoding is stateless.

If s is not NULL, the mbtowc() function returns the number of
consumed bytes starting at s, or 0 if s points to a null byte, or -1
upon failure.
If s is NULL, the mbtowc() function returns nonzero if the encoding
has nontrivial shift state, or zero if the encoding is stateless.

This page is part of release 5.00 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2019-03-06 MBTOWC(3)